Direct Wiring of Eutectic Gallium–Indium to a Metal Electrode for Soft Sensor Systems

Abstract

For wider applications of liquid metal-based stretchable electronics, electrical interface has remained a crucial issue due to its fragile electromechanical stability and complex fabrication steps. In this study, a direct writing-based technique is introduced to form the writing paths of conductive liquid metal (eutectic gallium−indium, eGaIn) and electrical connections to off-the-shelf metal electrodes in a single process. Specifically, by extending eGaIn wires written on a silicone substrate, the eGaIn wires were physically connected to five different metal electrodes, of which stability as an electrical connection was investigated. Among the five different surface materials, the metal electrode finished by electroless nickel immersion gold (ENIG) was reproducible and had low contact resistance without time-dependent variation. In our experiments, it was verified that the electrode part made by an ENIG-finished flexible flat cable (FFC) was mechanically (strain, ≤100%; pressure, ≤600 kPa) and thermally (temperature, ≤180 °C) durable. By modifying the trajectories of eGaIn wires, soft sensor systems composed of 10 sensing units were fabricated and tested to measure finger joint angles and ground reaction forces, respectively. The proposed method enables eGaIn-based soft sensors or circuits to be connected to typical electronic components through FFCs or weldable surfaces, using only off-the-shelf materials without additional mechanical or chemical treatments.

Liquid Metal–Based Soft Microfluidics

“…LM, on the one hand, possesses excellent electrical and thermal conductivities. On the other hand, distinguishing it from traditional metals, LMs exhibit good flowability, reconfigurability, and recoverability that can hardly be achieved by a solid metal . As the most familiar liquid metal in daily life, mercury (Hg) possesses its fatal drawbacks in volatility and toxicity, which extremely hampers its applications for practical usage.…”

“…On the other hand, distinguishing it from traditional metals, LMs exhibit good flowability, reconfigurability, and recoverability that can hardly be achieved by a solid metal. [5] As the most familiar liquid metal in daily life, mercury (Hg) possesses its fatal drawbacks in volatility and toxicity, which extremely hampers its applications for practical usage. Gallium and its alloys, such as EGaIn (eutectic alloy of gallium and indium) and Galinstan (alloy of gallium, indium and tin), are good replacement to overcome Motivated by the increasing demand of wearable and soft electronics, liquid metal (LM)-based microfluidics has been subjected to tremendous development in the past decade, especially in electronics, robotics, and related fields, due to the unique advantages of LMs that combines the conductivity and deformability all-in-one.…”

Liquid Metal Direct Write and 3D Printing: A Review

“…This reality underscores the importance of staying below the critical yield stress (i.e., keeping the left hand side of the equation equal to the right). Liquid metal direct writing has been used to make electrical contacts with graphene, silver, carbon nanotubes, copper, and other electrode materials [57][58][59][60] At its simplest, direct writing enables liquid metal features to be patterned directly on a surface in an arbitrarily complex 2D geometry. Two general approaches can be used to drive the direct writing process, i) shear and ii) volumetric control.…”

Advances in Liquid Metal-Enabled Flexible and Wearable Sensors

“…When exposed to the air, liquid metal will be oxidized immediately and form a layer of protective film. Unlike conventional liquid metal like mercury, gallium and its alloys is proven to be low-toxic and biocompatible, and has been introduced to biomedical area [55].Due to the unique properties of liquid metal, it has been vigorously studied in soft electronics, such as flexible conductors, antennas, transistors, electrodes, energy-harvesting devices, sensors, liquid robots and liquid computational systems [56][57][58][59][60][61][62][63][64][65][66][67][68]. In particular, liquid-metal sensors are designed on the basis of variable resistance and capacitance during stretching and bending.…”